A sudden spike in traffic which exceeds your planned capacity means that not only are your servers running at 100% but now they need to also dedicate a portion

A sudden spike in traffic which exceeds your planned capacity means that not only are your servers running at 100% but now they need to also dedicate a portion of their now-fully-committed capacity to syncing their data to the new server(s). You might be able to bootstrap this process partially by priming a new server from backups and reducing the load on operational servers, but it's not a free lunch.